The invention relates generally to the field of non-invasive imaging and more specifically to the field of computed tomography (CT) imaging and inspection systems. In particular, the invention relates to a technique for correcting scatter from digital radiographs acquired via volumetric computed tomography (VCT) systems.
Inspection of objects is of vital importance in manufacturing and repair industries. Various types of inspection systems, such as computed tomography (CT), coordinate measuring machines (CMM), laser-based profilometry, light gauge, infrared and others, are used in industrial inspection processes for a wide variety of applications. For example, these inspection systems may be used for measuring dimensions or for identifying defects in manufactured parts, such as turbine blades. Each of these inspection systems has its advantages and disadvantages. Modalities such as CMM and laser-based profilometry typically measure external surfaces with high accuracy, but cannot measure internal features unless the object is cut open. To date, CT is the most versatile of the measurement/inspection systems for revealing both the internal and external structures of industrial parts in a non-destructive manner. Because of their ability to provide internal as well as external measurements, CT based techniques may facilitate processes such as reverse engineering, rapid prototyping, casting simulation and validation, tire development, first article inspection, ceramic porosity inspection, process validation, parts qualification and defect detection, among others. However, CT based techniques may also have certain limitations, which may deter their widespread use.
For example, volumetric computerized tomography (VCT) imaging for industrial applications (e.g., imaging of metallic parts) typically provides unsatisfactory images having image artifacts due to radiation-matter interaction based artifacts, scanner based artifacts, reconstruction techniques based artifacts, and so forth. The radiation-matter interaction based artifacts may further include beam hardening artifacts and artifacts due to x-ray scatter radiations. Scatter radiation in the projection images reduces the contrast of the projection images, produces degradation of or blurs sharp features of the object in the generated volume images, and reduces the accuracy of metrology applications and the detectability of smaller features. Scatter radiation is a strong function of the imaging parameters such as the object under imaging, beam spectrum used, geometrical distances, and the surrounding medium. Due to various dependencies in the imaging parameters, an accurate estimation of the scatter signal content in projection imaging is challenging. Physics-based models are often used for predicting scatter content in x-ray images, however they are time consuming and predict only scatter arising out of the object under scanning, provided the material properties are known.
There exist different techniques for scatter measurement and scatter correction in acquired projection images. For example, one popular scatter measurement technique employs a beam stopper located between the radiation source and the object being scanned in a VCT system to measure the scatter at a corresponding location. However, most currently known techniques primarily address the object scatter and involve time-consuming computer simulations.
As manufacturing tolerances become tighter, there is a corresponding increase in the demands for metrology techniques for maintaining the tolerances. The need for quality and performance testing has become an integral part of the production or manufacturing process. Thus, in order to improve CT inspection accuracy and efficiency, more effective methods are needed for removing scatter radiation related artifacts. Accordingly, a need exists for a measurement technique that is relatively inexpensive, versatile and programmable for different applications and that requires low maintenance.
It is therefore desirable to provide an improved scatter measurement and correction technique that accurately measures scatter radiation in the projection images and removes the same from the projection images, thereby improving the VCT image quality. It is also desirable that the improved scatter measurement and correction technique is inexpensive and computationally efficient and time efficient, thereby increasing the throughput of the VCT system.